Animating multiple instances of a single armature within a single animation
Yes you could do that. The easiest way to do this is by actually duplicating your model and armature attached to it. So you will have ArmatureA (using ModelA) and ArmatureB (using ModelA).

They both need to physically exist in your scene. I usually tend to place them right behind the cameras, so every group/instance/reference/dupli object/particle or animation track that will reference them will not get them rendered accidentally.

So now you go to the NLA, Select ArmatureA and do animation in the graph editor. On the NLA save the animation clip.

Now, when you want to apply that animation to ArmatureB,  it will accept it only after you freeze the clip (it's like making a new fake user material or a new + material). Once you have created (frozen) your NLA track, on the property name of the stored track, select your ArmatureB. Now it should move.